I like this one so much it might end up becoming a novel.
Hinges Between Days
Amanda Baker looks around the half-demolished Victorian mansion, thinking this would be a great place for a party as she considers how to celebrate the end of high school. And then she sees a light from under a door, which should open onto a brick wall now... but it opens. She sees a room that should no longer exist, fully decorated and lit by oil lamps. And she hears music from beyond. She steps further inside, hears someone coming and runs back through - finding herself in the house as it is now again. But looking back in, the room from the past is still there...
A few days later, something points the group to the manor. Investigators of strange events could hear about disappearances inside the house, going back to the last family to live in the house over a century before. This could also be a good introduction to a time-travelling game for "civilian" characters. A TARDIS would be drawn to a disturbance in time, a UNIT or Torchwood team should have a way of detecting those as well.
And something else has detected it. A nameless monster, a Beast Between Moments, hunts in gaps in time. It may be the cause of the disappearances over the years since the house was abandoned...
As often happens in stories about finding a door that leads back in time (examples: Tom's Midnight Garden and Moondial), Amanda has become fascinated by this lost world she can now visit, by the people there and their long-forgotten lives. Perhaps one of them is the key to why the timeslip started?
Unfortunately, unlike the heroes of most such stories, Amanda has told her friends, and they've told their friends, and so on. It could look like a Skins party on one or both sides of the timeslip by the time the group arrive to try and close it.
Things To Do: Stop the crosstime contamination, banish the Beast and find a way to break the connection.
Antagonists: The Beast Between Moments, drawn to this crack in time like a predator to a watering hole. For now, it is content to take the careless, but if the timeslip is threatened it will move to defend its territory.
Action Scenes: A chase across two centuries that has to be kept quiet in both.
Problems: So you have dozens of modern teenagers crashing a party in the 1890s. Add an only-visible-sometimes monster into the mix, and you've got a recipe for total chaos. And if you really want to complicate matters, Amanda has fallen in love with the handsome young son of the house's 19th Century owners. Can you break two hearts for the sake of the timeline?
Things To Prepare For: If you want to find a suitable map, consider a "before and after" version, where some rooms have been demolished in the present, doors are locked in one time and off their hinges in the other - and anything affecting the house in the past will appear in the present, so imagine if someone starts a fire...
Continuing the Adventure: The Beast knows the group's scent. And there are now potentially dozens of people on both sides of the timeslip who know about time travel. And maybe a couple so in love that time itself won't stop them.